Sampling requirements for estimating the bycatch of Pacific halibut (Hippoglossus stenolepis) in the on-bottom trawl fishery in Hecate Strait / by J. Fargo, R.D. Stanley and C.M. Hand. : Fs97-4/2135E-PDF

"We investigated the precision of the bycatch estimate for halibut in the mixed species trawl fishery in Hecate Strait, derived from stratified mean CPUE. Using data collected from a 1981-82 observer program, we estimated mean CPUE for halibut and its variance for three areas of differing halibut habitat and density. From these estimates, we derived the relationship between precision of the estimate of the stratified mean CPUE and sample size (number of tows observed). Assuming that the sampling of tows is random, we found that precision of the stratified mean CPUE decreased from +50% to ±22% when the number of observations increased from 30 to 200 tows. Precision decreased to ±10% with a sample size of 850 tows. We estimate that a random sample of 20 commercial trips from this fishery, about 320 tows, of which +80% (250) could be observed, will yield an estimate of the stratified mean CPUE for halibut, for Hecate Strait, with a precision of +20% 19 times out of 20. We caution that because tows within a trip are not independent, and because we can expect considerable among trip variation, bycatch estimates based on a limited number of trips (< = 5) could be seriously biased"--Abstract, page iii.

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